1. Field of the Invention
The present invention relates to an image forming device, an image forming method and a storage medium.
2. Description of the Related Art
An electronic photograph system is known as an image printing system used in a color image forming device such as a color printer or a color copying machine. The electro photograph system is designed to form a latent image on a photosensitive drum using a laser beam and develop the latent image by a charged color material (hereinafter, refer to a toner). Printing of an image is carried out by transferring and fixing the developed image by the toner onto a transfer paper.
In recent years, a color image forming device of a tandem system has been increasing, which is provided with developing machines and photosensitive drums each number of which is the same as the color number of the toner for achieving a high speed in image forming and sequentially transfers images in different colors onto an image carrier belt or a print medium. It is known that in this color image forming device of the tandem system, there exist plural factors causing a registration shift, and therefore, various measures against the respective factors are proposed.
One of the factors is unevenness or a mounting position shift of a lens in a deflection scan device and a mounting position shift of the deflection scan device to a color image forming device body. Caused by this position shift, an inclination or a curve in a scan line is generated and a degree of the curve (hereinafter, refer to a profile) differs in each color, creating a registration shift.
The profile has a characteristic which differs in each image forming device, that is, each printing engine and further, in each color. Examples of the profile are shown in FIGS. 13A, 13B, 13C and 13D. In FIGS. 13A, 13B, 13C and 13D, a lateral axis shows a main scan direction position in the image forming device. A line 1300 linearly expressed in the main scan direction shows an ideal characteristic free of a curve. In addition, each of a line 1301, a line 1302, a line 1303 and a line 1304 shown in a curve shows a profile in each color. The characteristic of cyan (C) is shown in the line 1301, the characteristic of magenta (M) is shown in the line 1302, the characteristic of yellow (Y) is shown in the line 1303 and the characteristic of black (K) is shown in the line 1304. A longitudinal axis shows a pixel shift amount in a sub scan direction to the ideal characteristic. As shown in FIGS. 13A, 13B, 13C and 13D, a changing point of the curve differs in each color, and this difference appears as a registration shift in the image data after fixed.
As a method of handling the registration shift, Japanese Patent Laid-Open No. 2002-116394 describes a method in which in an assembling process of a deflection scan device, a magnitude of a curve in a scan line is measured by using an optical sensor and a lens is mechanically rotated to adjust the curve in the scan line, and thereafter, the lens is fixed by an adhesive material.
In a method according to Japanese Patent Laid-Open No. 2003-241131, in a process of mounting a deflection scan device to a color image forming device body, a magnitude of an inclination in a scan line is measured by using an optical sensor. Thereafter, the deflection scan device is mechanically inclined to adjust the inclination in the scan line, and then, is mounted to the color image forming device body.
Further, each of Japanese Patent Laid-Open No. 2004-170755 and Japanese Patent Laid-Open No. H04-326380 (1992) describes a method in which magnitudes of an inclination and a curve in a scan line are measured by using an optical sensor and the bit map image data are corrected to cancel out the magnitudes, forming the corrected image. In this method, since the registration shift is electrically corrected by processing the image data, a mechanical adjustment member or an adjustment process on assembly becomes unnecessary. In consequence, it is possible to downsize the color image forming device and also the registration shift can be handled less expensively than in each method described in Japanese Patent Laid-Open No. 2002-116394 and Japanese Patent Laid-Open No. 2003-241131. This electrical correction of the registration shift is classified into correction of one pixel unit and correction of less than one pixel. The correction of one pixel unit, as shown in FIG. 14, offsets the pixel per one pixel unit in a sub scan direction in accordance with correction amounts of the inclination and the curve. It should be noted that in the flowing description, the offset position refers to “a line changing point”. That is, in FIG. 14(a), P1 to P5 correspond to line changing points.
The correction of less than one pixel, as shown in FIGS. 15A, 15B, 15C, 15D and 15E, is made in such a manner as to adjust a tone value of a bit map image data with a pixel before or after a sub scan direction by a laser light volume adjustment or PWM (Pulse Width Modulation). That is, in a case where the scan line is curved in an upper direction according to a profile characteristic as shown in FIG. 15A, the bit map image data before the tone correction is handled in the sub scan side in a direction reverse to a direction shown by the profile. By performing the correction of less than one pixel with this method, it is possible to eliminate an unnatural step in a line changing point boundary generated due to the correction of one pixel unit to achieve smoothness of an image.
In addition, Japanese Patent Laid-Open No. 2006-301030 describes a method of moving a center of gravity by position shifting per pixel unit. This method moves the center of gravity by controlling a cycle of a pixel and can move the center of gravity without adjustment such as PWM.
In the above conventional technology, however, a pixel position of less than one pixel is shifted by laser power modulation using PWM or current control at laser scanning to execute correction processing, thus removing the step of less than one pixel. Therefore, an image in which the density is expressed with roughness and closeness of a microdot, for example, one dot results in an event that the density is expressed with plural intermediate dots (two or more dots), leading to instability of dot formation.
FIGS. 16A, 16B and 16C show a state of a center-of-gravity movement using intermediate dots by laser power modulation. That is, FIGS. 16A, 16B and 16C show a state where a scan line is gradually shifted from right to left in that order. The curves shown in a broken line show exposure images which are generated by one laser scan and the curve shown in a solid line shows an exposure image including an influence of the neighboring laser exposure. This processing performs an interpolation center-of-gravity movement based upon a pixel shift amount from the laser scan position. The center-of-gravity certainly seems to gradually move to the left side while storing the integral value, but the generated forms of dots are not necessarily identical with each other, a difference of which may appear possibly as a change in density. Therefore, even if the density is saved based in light of a signal value or an integral light volume, the outputted image may not possibly maintain the density.
That is, even if the image subject to light emission by a light volume of 0.3 is adjacent to the image subject to that by a light volume of 0.7, it is difficult to realize the order of the same density as that subject to light emission by a light volume of one, and the possibility that the center-of-gravity is shifted by 0.3 is low. This means that the density save based upon the center-of-gravity shift by using intermediate dots is difficult.
The similar phenomenon also occurs in a line width of a thin line, and even if the same line width is realized in light of a signal value around processing the correction of less than one pixel, the line widths in the outputted thin lines possibly differ visually with each other.
FIG. 17A shows an example of correcting a center of gravity by position shifting per one pixel unit. In this case, the density tends to be easily saved, but in a case of desiring to draw a one-dot inclination line as shown in FIG. 17B, a region of not being scanned by a laser may be produced in an image formed as shown in FIG. 17C, thus generating a blank. In consequence, the inclination line is drawn in a broken line, which causes an image defect (broken line problem). When the drawing is made in this way, convex and concave portions may be visible depending on a level of resolution in image forming.